Don't keep trees for modules with syntax errors.
Add a test for get_update().
Fix minor refactoring error.
Fix visit_Raise on Python 3 and add a test.

Also raise test coverage of symbols_test a bit.
Fix SymbolIndex.add() on Python 3.

On Python 3, SymbolIndex and float instances cannot be compared.
Merge pull request #4 from birkenfeld/local

A bit more coverage improvement, fix resulting test failures